base timing keeps advancing by itself...

Ok I just identified a really weird problem that has me scratching my head.

Set base timing to 10 with engine warm, vac advance unplugged. Drove all day, ran great. Checked timing on shutdown with engine fully hot... base timing was now 14. I have a mark on the carb plate with an indicator stick I put on the dizzy, confirmed that the distributor is locked down and didn't move. Engine was idling at higher RPM, corresponded to increased timing so I don't think dampener slipped or anything like that. Bolts are tight.

Reset timing back down to 12 base with vac can unplugged and engine fully hot. Drove again next day.. ran great, but about halfway into the day I noticed detonation rattle around 2000-2400 rpm that should not have been happening... and the car was suddenly idling at 1000 rpm which is way too high. Checked timing again... 22 degrees base timing now. Indicator marks show distributor has not moved. Busted timing back down to 12 btdc and idle returned to normal 750-800.

What the hell is going on with this thing? All my theories don't check out...
1) though maybe dizzy springs are weakened some how and it is bleeding in base timing at idle... but that would not effect the WOT detonation I am getting around 2000-2500 that indicates all in timing is way to much (entire curve has been shifted by 10 degrees!)
2) distributor is locked down TIGHT.
3) Vac can locks distributor trigger (pertronix) into position... but everything looks good in distributor no slop and V-Can is brand new and working properly.

What the hell is going on with this thing... I think all my past issues with detonation are due to this movement... when base timing is actually 12-16 degrees, the car runs fantastic. But when I set it warm.. it moves by 4-8 degrees advanced after an hour or 2 of driving... it has done this twice now. Now I realize there is no issue with my curve, plugs, fuel, any of that... my base timing just keeps increasing on its own somehow.